The present invention relates to precision rocker mills or pilger mills of the type which produce tubes from hollow metal workpieces.
A rocker mill is disclosed, for example in Kondoh U.S. Pat. No. 4,562,713 and in copending U.S. application Ser. No. 07/297,431 filed Jan. 17, 1989, the disclosure of which is incorporated herein by reference. Such mills typically include a movable rollstand which is reciprocated along a hollow workpiece. The rollstand includes a pair of grooved roll dies which define a nip through which the workpiece is passed so as to be radially compressed. A mandrel extends coaxially through the workpiece to radially support the inside of the workpiece.
The grooves of the roll dies are of progressively narrowing width in the circumferential direction. Thus, by rotating the roll dies during each reciprocable stroke of the rollstand, the workpiece is subjected to a progressively increasing radial compression whereby the diameter of the workpiece is progressively reduced.
At the end of each stroke it is necessary to reposition the workpiece for the next stroke. Accordingly, the workpiece is advanced longitudinally by a selected linear distance, while being rotated by a selected angular distance. Longitudinal advancement of the workpiece is accomplished by a longitudinally movable feed carriage which has a chuck or clamp that operatively engages the outer periphery of the workpiece. A feedscrew extends longitudinally of the mill and passes through a stationary threaded nut carried by the carriage. A motor is provided at one end of the feedscrew which periodically rotates the feedscrew in order to longitudinally advance the carriage and thus also the workpiece.
Rotation of the workpiece is accomplished by a plurality of motor-driven rotary clamps located in front of and behind the rollstand. The workpiece, which passes longitudinally through the rotary clamps, is gripped by the clamps, and the clamps are rotated by a selected angular distance. Likewise, a rod which is affixed to the mandrel to form a rear extension thereof passes through a clamp which grips the rod and rotates it in order to rotate the mandrel by the selected angular distance.
The above-described mechanism for longitudinally and rotationally moving the workpiece and mandrel involves an elaborate system comprised of the feedscrew and a gear-laden drive train for rotating the feedscrew and actuating the various clamps. That system is characterized by a high inertia due to its large overall weight and thus exhibits a slow reaction time upon start-up and stopping whereby it is difficult to reposition the workpiece rapidly and precisely. As a result, the rate of production and the quality of the tubes produced can be adversely affected.